I have two schemas A and B. Both are on same database running on Oracle 11.2.0.1. I have 3 tables in A from where I am loading data into B into one table. To do so I am using a simple SQL Query. After providing all the grants and after creating the required synonyms, I am able to run the query and it is inserting the correct records into the target table in B.
Now, I would like to generate a diagram of this operation. I am in serious need of a easy to use tool which can be used to create a diagram showing this data flow with details like which columns are getting selected from A to B by my SQL Query.

Oracle's SQL Developer Data Modeller apparently has some kind of Data Flow Diagram capability. I have not looked into it yet myself, but from what I have read it sounds appropriate, although I doubt it would display at the column level. You could reverse engineer the tables into the model and then create the data flow manually. Try it out.